The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present disclosure may or may not address one or more of the background issues.
With the current trend in the use of capacitive touch screen user interfaces in mobile telephones, the tactile feedback that comes with changes in finger pressure as on a conventional key-dome key pad has to a great extent been lost. To overcome this, a method of detecting the force applied to the screen in conjunction with the finger position is required. One solution is to use a semiconductor piezoresistive type force sensor such as the Hokuriku HFD-500. This is a resistive bridge which changes value linearly in proportion to the force that is applied to the silicon substrate. The sensor is mounted on the telephone's printed wiring board (PWB) and the force is transferred to the silicon from the screen mechanically.
The voltage differential from the force sensor is the product of not only the finger pressure (an external force), which represents only a small component of the total force, but also a much greater and unknown compression force (an internal force) which is a result of the glass pressing down on the sensor when the phone is assembled. Also, the device has a broadly specified offset voltage which may vary anywhere within the offset limits. All of these factors are subject to variation over time due to, for example, changes in temperature and environment and may also drift with aging.
The standard solution is to amplify the signal using a simple linear amplifier and remove the d.c. offset in software by filtering. The problem with this method is that because the desired signal representative of an external force is so small in proportion to the offset caused, at least in part, by an internal force, the amplifier gain would be defined by the point at which the maximum total signal would start to be limited. However, as the maximum total signal is unknown, the gain would have to be sufficiently small to give a comfortable margin. With such a small gain, the required signal representative of an external force may not be large enough to be resolved accurately.